Integrated process and related system for obtaining energy from waste with low investments and high thermoelectric yields

ABSTRACT

A process is described for obtaining energy from waste, comprising the following phases: a) bio-drying of municipal solid waste (MSW) to transform it into refuse-derived fuel (RDF), a dry, homogeneous material with piece size of around 20-30 cm, known by the name of RDF; h) compacting of the material obtained from phase a) into bales or BIOCUBr and storage of the BIOCUBI® in bioreactors; c) activation by wetting with water of the bioreactors to produce biogas by anaerobic digestion; d) combustion at the start of the material obtained from phase a) (RDF) and subsequently of the residue already digested in the bioreactors, and therefore not biodegradable, in a waste combustor provided with a system of purification of combustion gasses and production of superheated steam at approximately 400° C. and pressure of around 70 bar; e) combustion of the purified biogas in a conventional boiler provided with re-superheaters for raising the temperature of the steam produced by the waste combustor by approximately 100° C.; f) use of the steam produced in this way in a turbine coupled with an alternator for the production of electrical energy. The invention also relates to a system for the implementation of this method.

FIELD OF APPLICATION

The present invention relates to an integrated process of known technologies such as the BIOCUBI® process for the bio-drying and stabilisation of MSW, anaerobic digestion in bioreactors for the production of biogas and incineration of the residual fraction of the bioreactors.

The present invention discloses a new process and method, and a related system, for integrating these technologies in order to increase thermoelectric yields and hence with maximisation of the production of electrical energy.

STATE OF THE ART

Municipal solid waste (MSW) from selective collection, after having been compacted, is generally dumped in controlled landfills with previously waterproofed bases according to the prior art (conventional landfill).

The growing production of waste, the consequent need for further spaces for controlled landfills and the problems of environmental impact of traditional landfills have encouraged the development of disposal methods as alternatives to dumping in conventional landfills.

Among these we can mention the incineration of the original MSW or a new technology, which is now becoming widespread in Italy and the whole of Europe, and which consists of conversion of the MSW into inert materials which can be compacted and dumped without environmental impact or can easily be transported and fed for combustion to produce electrical energy.

The patent EP 0706839 of the same Applicant shows how the MSW can be treated by aerobic digestion of the putrescible material with low heating value to evaporate the water and obtain a fuel with a high heating value to be fed for combustion.

Refuse-derived fuel (RDF), which we will refer to as RDF, can be fed into grate combustors or, refined further, can be used in fluidised bed combustors or in cement works.

This patent also shows how, without combustion systems, said fuel is odourless, stable and free from pathogens and, when dumped, does not therefore have the problems of environmental impact of conventional MSW landfills.

The patent EP 1386675 shows in detail a possible version of the plant and of the system of control and management of the process.

It is however known that combustion of waste and also of RDF has technological limitations concerning the combustion yields, caused mainly by the presence of chlorine and hence of chlorides that are corrosive to the superheaters which advise operation with low temperatures of the steam in the boiler and typically of around 400° C., with consequent low electrical energy yields compared to the combustion of fossil fuels and typically of around 25%.

The small scale of the local incinerators that limits the yields and increases investments per unit of fuel burnt increases this disadvantage.

To improve the yields, in patents EP 1382806, EP 1416223, EP 1428987 and EP 1430952 the Applicant has in turn proposed the combination of large waste combustors with a conventional thermoelectric plant, so as to use in the best possible way the steam produced while maintaining the limit of 400° C. on the incinerator super heaters.

The thermoelectric yields rise in this way to around 32% although, however, the presence of a power plant available for the integration is required.

It was later found that the RDF, dumped in a non-conventional landfill, if wetted thoroughly, can be activated on demand for the production of biogas, showing very high productivity levels compared to those of a conventional landfill.

The patent EP 1520634 of the same Applicant gives the results obtained directly at an unconventional landfill, introducing the concept of bioreactor, which has already been demonstrated on an industrial scale at the site at Corteolona (PV) with the production of 4 MW of electrical energy using internal combustion engines.

The patent application WO2005102547 of the same Applicant provides an integration of the bio-drying plant for the production of RDF and a refinement process to produce a fuel with high heating value with the bioreactor for producing electrical energy.

This integration is partial, since said fuel has then to be transported to a combustion plant: however a first industrial realisation is already available at the site of Villafalleto (CN).

It should finally be noted that, among the technologies presented to date, the activatable bioreactor represents the most economical solution from the point of view of investments, followed immediately by bio-drying for producing RDF while combustion is by far the most expensive one.

Therefore the first two technologies are immediately available, have advantages from the point of view of environmental impact and social acceptability and therefore represent a concrete and easily implemented solution to the problem of waste.

The combustion of the waste can be performed subsequently when the first two stages of treatment and storage are completed.

DESCRIPTION OF THE INVENTION The general object of the present invention is that of eliminating the disadvantages described above by making available a method and a related system which allows maximum exploitation of the energy content of the waste with minimum investments and above all by staggering these investments in time.

A further object is that of treating waste, transforming it into RDF and therefore making it inert and ensuring safe storage of the fuel produced in this way in the bioreactors.

A further object is that of making available a method for recovering from the activatable bioreactors energy at a high level in the form of purified biogas without chlorinated substances and therefore such as to be used to improve the heat content level of the steam produced by the combustion of RDF or better from the residual fraction of the bioreactors.

An additional object is that of reducing the load of contaminated waste at the combustor, reducing at the same time the investments and recovering the inefficiencies of the combustor by means of the purified biogas.

These objects, and others that are to be described in greater detail herein below, are achieved by a method and by a process, which considers the entire life cycle of the waste and integrates the available technologies synergically.

The method according to the invention has the features of the annexed independent claim 1.

Advantageous embodiments are described in the dependent claims.

The method therefore constitutes an alternative to the traditional system of combustion in incinerators of the MSW and of the fuels derived therefrom and to their dumping.

The method also constitutes an alternative to those already implemented industrially or proposed by the same Applicant as demonstrated in the prior art.

Referring to FIG. 1, the process can be schematised as follows:

-   a) Biostabilisation by the method described in EP 0706839 and EP     1386675 with the object of transforming MSW into a dry and     homogeneous material with piece size of around 20-30 cm and which     can be treated easily (RDF). -   b) Compacting of RDF into bales or BIOCUBI® with size of around     1×1×2 in and storage of the BIOCUBI® in the bioreactors as described     in EP 1520634 and WO2005102547. -   c) Activation of the bioreactors by wetting with water to produce     biogas: the sizes of the bioreactors are such as to form large     reservoirs of biogas and the times for the completion of the     anaerobic process are reduced so as to make available in a shorter     time, compared to a conventional landfill, (5-6 years) the     non-biodegradable residue formed by inert and plastic. The biogas     produced is purified of the possible residual traces of chlorinated     products by means of known techniques, e.g. passage over active     carbon beds. -   d) Combustion at the start of RDF and subsequently of the residue     already digested in the bioreactors, and therefore     non-biodegradable, in a waste combustor equipped with a system of     purification of exhaust gasses and production of superheated steam     at approximately 400° C. and pressure around 70 bars. -   e) Combustion of the purified biogas in a conventional boiler     equipped with re-superheaters for raising the temperature of the     steam produced by the waste combustor by approximately 100° C. -   f) Use of the steam produced in this way in a turbine coupled with     an alternator for the production of electrical energy.

A further possible integration presented in FIG. 1 is represented by the use of the contaminated air released by the process of bio-drying for the combustion of the waste.

In this way it is possible to eliminate, wholly or in part, the treatment of the air released by bio-drying.

Bio-drying is described in detail in the aforementioned patents and serves to dry and stabilise the waste and produce substrates that can be activated and are particularly reactive for the anaerobic digestion in the bioreactors.

To sum up, the waste and in particular the MSW is fed in a closed space where it is ground roughly and arranged in layers on a floor with holes which is divided into sectors through which the air is passed: aerobic digestion of the putrescible part develops heat which evaporates the water which is removed with the air flow.

Starting with 100 kg of MSW having a humidity content of around 36% and a lower heating value (LHV) of 2200 kcal/kg, 75 kg of RDF are obtained with a humidity content of around 19% and an LHV of around 2900 kcal/kg.

The product obtained in this way is odourless and stable, can be compressed into bales or BIOCUBI® with density of around 700 kg/m³ and placed in the bioreactor.

The BIOCUBI® can be produced on site or easily transported from production systems distributed in the territory.

The bioreactor is also divided into sectors as described in WO2005102547 so as to activate it by injecting water into each sector in sequence over the years and have a continuous production of biogas and at capacity a tank and continuous availability of exhausted material at the end of the life of each sector.

In time it may be possible to reduce the use of water, replacing it with percolates or other liquid effluents containing digestible substances and therefore able to form an additional source of biogas.

The bioreactors are completely covered by a polymeric membrane, which ensures the containment of the biogas produced and are equipped with internal manifolds for its collection.

They therefore represent large tanks suitable for guaranteeing availability of the gaseous fuel for the process.

When a sector ends production, the biodegradable portion being exhausted and the part composed of soil, plastic and part of the wood remaining, air is aspirated or blown so as to eliminate part of the residual water and obtain a humidity content of between 20 and 25%.

Optionally, in order to accelerate evaporation of the water and/or obtain low residual humidity of the waste, hot air or mixtures of air/gasses from the combustion could be used (not shown in the drawings).

Again on the basis of 100 kg of MSW initially, the product of the bioreactors is typically 59-63 kg of exhausted material having an LHV of around 2700-2600 kcal/kg.

15-20 kg of biogas are also produced with a content of methane of around 45-55% in volume, according to the nature of the initial material, and having an LHV of around 2900 kcal/kg.

At this point the significant reduction in the solids fed to the combustor and the improvement in the energy content of the fuels to be burnt compared to the initial MSW should be noted.

The combustor of the solid waste has a reduced load and with improved efficiency and the purified biogas is used partially or totally in an additional boiler for re-superheating the superheated steam, allowing the limit of 400° C. of conventional combustors to be exceeded.

Referring to the diagram of FIG. 2, the combustor is for example composed of a grate furnace provided with a boiler for the production of superheated steam at 400° C. and 70 bar, equipped with the combustion gasses purification line formed by a series of cyclones for abating the substantial part of the solids conveyed, a cooling tower for controlling the temperature, a reactor fed with lime and active carbons for the neutralisation of the residual acidity and the adsorption of the micropollutants and a bag filter for retaining the fine dust before emission at the stack.

The superheated steam generated in the boiler, and possibly a share of control water, is sent to the superheater fed with biogas where the temperature of the steam is raised by around 100° C. according to the biogas available.

This steam, expanded in the turbine, allows a yield of the thermal cycle of FIG. 2 of around 32% to be obtained and therefore similar to that which can be achieved with the waste combustor coupled with a thermoelectric plant.

The results obtained with the invention claimed here are given in the following examples of application, based on the experience of industrial bio-drying systems, on experiments of a system for the production of biogas and on available waste combustors.

EXAMPLE

The intent is to build a large waste combustor having a supplied thermal power of 110 MWt such as that in FIG. 2, using the innovative cycle, which is the object of the present invention, shown schematically in FIG. 1.

The combustor is fed with 34 t/h of solid waste obtained from the bioreactor starting from 43 t/h of RDF obtained in turn through bio-drying from 58 t/h of MSW.

The boiler of the combustor produces, dispersed heat accounted, 122 t/h of steam superheated at 400° C. and 70 bar which is sent to the re-superheater fed with 6.8 t/h of biogas produced in the bioreactor.

128 t/h of superheated steam are produced at 492° C. of which 6 t/h of steam from temperature control added water.

This steam, expanded in the turbine, generates, including the yields of the machines, 35.5 Mwe with a yield of the thermoelectric cycle of over 32%.

The process plan proposed therefore represents a valid alternative to the integration of the waste combustor with a thermoelectric plant if this plant is not available on site.

The large reduction in the solid waste to be burnt compared to the combustion of RDF and above all of MSW should be remarked, thanks to the production of the biogas that, in the process, which is the object of the present invention, serves to raise the heat level of the steam and therefore the yields of the entire cycle. 

1. Process for obtaining electrical energy from waste, comprising the following steps: a) bio-drying municipal solid waste (MSW) into refuse-derived fuel (RDF), said RDF comprising a dry, homogeneous material with piece size of around 20-30 cm; b) compacting the RDF obtained from step a) into bales and storing the bales in bioreactors; c) activating the bioreactors by wetting the bales with water to produce biogas and a non-biodegradable residue through anaerobic digestion; d) combusting the non-biodegradable residue of step c) in a waste combustor provided with a system for combustion gasses purification to produce superheated steam at approximately 400° C. and pressure of around 70 bar; e) combusting the biogas in a conventional boiler equipped with re-superheaters for raising the temperature of the steam produced by the waste combustor by around 100° C.; and f) directing the steam produced in steps d) and/or e) in a turbine coupled with an alternator for production of electrical energy.
 2. (canceled)
 3. Process according to claim 1, wherein said bio-drying of step a) further comprises feeding the MSW in a closed space, grinding the MSW roughly and placing the ground MSW in heaps on a floor with holes, the MSW being divided into sectors through which a current of air is passed, wherein aerobic digestion of putrescible part develops heat, said heat causing evaporation of water which is removed by the current of air.
 4. Process according to claim 1, wherein said bales have a size of around 1×1×2 m and density of around 700 kg/m³.
 5. Process according to claim 1, wherein said bales are produced on site or transported from production stations distributed in the territory.
 6. Process according to claim 1, wherein said bioreactors used in step c) are divided into sectors forming large storage tanks, wherein said sectors are activated by injecting water into each of them, so as to provide a continuous production of biogas and provide, at the end of the life of each of said sectors, continuous availability of non-biodegradable exhausted material, wherein said non-biodegradable exhausted material is made up of inert and plastic.
 7. Process according to claim 6, further comprising blowing or aspirating air at the end of the life of each of said sectors so as to eliminate part of the residual water and obtain a low content of humidity, between 20 and 25%.
 8. Process according to claim 7, further comprising using hot air or mixtures of air/gasses from the combustion to accelerate evaporation of the water and/or obtain low residual humidity of the waste.
 9. Process according to claim 1, wherein the biogas produced in step c) is purified of possible residual traces of chlorinated products by passage on active carbon beds.
 10. Process according to claim 1, wherein said combustion gasses purification system comprises a series of cyclones for abating the substantial part of he solids conveyed, a cooling tower for controlling the temperature, a reactor fed with lime and active, carbons for neutralising the residual acidity and adsorption of the micropollutants and a bag filter for retaining the fine dust before emission at the stack.
 11. Process according to claim 1, wherein the steam, expanded in the turbine, allows a yield of the thermal cycle of around 32%.
 12. System according to claim 1 further comprising: a bio-dryer for drying and stabilizing the MSW for producing RDF, said RDF being reactive and activatable for anaerobic digestion in bioreactors; means for the compacting of said RDF into bales; at least one bioreactor fed with said bales for producing biogas; a waste combustor provided with a combustion gasses purification system and production of superheated steam; a boiler for combustion of purified biogas provided with re-superheaters for raising the temperature of the steam produced by the waste combustor; a turbine coupled with an alternator for the production of electrical energy.
 13. Process according to claim 1, wherein contaminated air is produced at end of step a)
 14. Process according to claim 13 wherein said contaminated air is used for the combusting in step d).
 15. Process according to claim 1, wherein the RDF of step a) is combusted in step e). 